This invention relates to a surfactant system for recovery of oil from a subterranean reservoir.
It has long been known that the primary recovery of oil from a subterranean formation leaves a substantial amount of the oil still in the formation. This has led to the use of what is commonly referred to as secondary recovery or waterflooding wherein a fluid such as brine is injected into a well to force the oil from the pores of the reservoir toward a recovery well. However, this technique also leaves substantial amounts of oil in the reservoir, so-called residual oil, because of the capillary retention of the oil. Accordingly, surfactant systems have been employed either in place of the secondary recovery or more generally in a tertiary recovery process. One particularly suitable type of surfactant system is that which results in the formation of a microemulsion which is immiscible with the oil it is displacing. Such microemulsion systems are very effective in removing residual oil.
The surfactant systems employed to produce microemulsion type oil recovery basically contain at least three separate ingredients, brine, a surfactant and a cosurfactant. It is disclosed in Glinsmann, U.S. Pat. No. 4,125,156, issued Nov. 14, 1978, how to systematically optimize a system so as to give a combination of surfactant, cosurfactant and brine which produce low interfacial tension which is associated with good oil recovery.
Since these values are obtained as disclosed in said Glinsmann patent by plotting oil recovery, interfacial tension and volume fraction of equilibrated phases versus salinity, the salinity at which minimum interfacial tension is achieved has been considered the optimum (optimal) salinity. However, the minimum interfacial tension does not necessarily correlate exactly with maximum oil recovery. Also, different systems with theoretically similar characteristics exhibit drastically different results in practical applications in test cores or actual use. Finally, it occurs in some instances that a cosurfactant of the desired solubility simply does not exist or is not readily available at an economical price.